Method of reducing eye holing in lacquered tin-plate



March 7,1961 R, A NElsH 2,974,091

METHOD OF REDUCING EYE HOLING IN LACQUERED TIN-PLATE Filed Dec. 11, 1958 A A A my v O/LEI? DRIER WI TH HEXA VALENT ACID/C SOLUTION 0F CHROMIUM RE DUCING AGE/V T, 5.6., 0.2 L0% C/TR/C ACID ELECTROLYS/S WATER RINSE //V l/E N TOR RICHARD A. IVE/SH Affarn e) United States Patent METHOD OF REDUCING EYE Hill-LING IN LACQUERED TIN-PLATE Richard A. N eish, Snowden Township, Allegheny County,

Pa., assignor to United States Steel Corporation, a corporation of New Jersey Filed Dec. 11, 1958, Ser. No. 779,657

2 Claims. (Cl. 20437) This invention relates to the treatment of tin plate to minimize or eliminate the tendency to eye-holing when lacquered.

Most tin plate to be fabricated into containers is first coated by the fabricator on one side with lacquer, i.e., a solution of a synthetic resin in an organic solvent. It sometimes happens that the lacquer-coating solution dewets or draws back from certain points, under surface tension, leaving bare spots known as eye holes. This tendency is particularly troublesome because it develops only sometime after tinning is completed and thus cannot be readily detected by the tin plate manufacturer.

I have invented a novel treatment for tin plate whereby its eye-hoiing tendency on lacquering is greatly reduced or entirely eliminated. In general terms, my method consists in subjecting the plate briefly to the action of a reducing agent, after the usual chemical treatment following tinning, e.g., electrolysis as cathode in a solution containing hexavalent chromium. I may employ any convenient reducing agent but dilute solutions of citric acid or stannous chloride have been found to be particularly effective.

A complete understanding of the invention may be obtained from the following detailed explanation of the preferred practice which is illustrated schematically in the accompanying drawing, the single figure of which is a flow diagram.

Tin plate in the form of continuous strip S is delivered from a conventional electrolytic plating line shown at wherein the steel strip is cleaned, pickled, plated with tin, heated to brighten the deposited tin coating and finally quenched by a water spray. On emerging from line 10, the strip is passed through a stabilizing bath 11 where it is subjected to treatment for the formation of a protective film containing chromium compounds as'by electrolysis in a solution containing hexavalent chromium preferably as disclosed in Patent No. 2,606,866. The strip is next passed through a water rinse, such as bath 12. The process so far as described above is conventional. The stabilizing treatment efiected in bath 11 forms a protective chemical'coating on the tin surface deposited on the steel strip.

In accordance with my invention, 1 subject the strip of tin plate withthe protective chemical coating formed thereon as described, to the action of a reducing agent before it is finally oiled and sheared or coiled. A convenient way to effect this treatment is to pass the strip through a spray or bath 13 of, an acidic reducing solution, e.g a dilute aqueous solution of citric acid or stannous chloride. The acidity of the bath,'while not parof from 1 to 7, preferably about 3. An acidic solution is desirable to prevent leaving on the tin plate traces of I alkali which are detrimental to lacquering. The concentration of the solution may be from 0.2 to 1.0%,, and

preferably about .7%. The time of immersion in the solution need be only instantaneous, i.e., from 0.2 to 1.0 sec., preferably about .5 sec. bath may be from 60 to 200 F., preferably about F.

After passing through bath 13, the strip is passed through a drier 14 and then through a spray oiler 15, before being recoiled at 16. Squeegee rolls are preferably provided at the exit end of bath 13 to prevent excessive drag-out of the reducing solution.

The product made in accordance with my invention is remarkably free from eye holing tendency on lacquering, i.e., the dewetting tendency of the lacquer as applied. The treatment descirbed also has the effect of materially improving the adherence of the lacquer under shockand' largely used in making containers for goods for human consumption.

Although I have disclosed herein the preferred practice of my invention, I intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

1 claim:

1. A method of making tin plate which consists in electrodepositing .a coating of tin on a sheet-steel base, forming a protective film containing chromium com-' pounds and at least a residual amount of free hexavalent chromium over the tin coating and then treating the tin plate with an aqueous solution containing from .2 to 1% of a reducing agent effective to reduce any free hexavalent chromium thereon to trivalent chromium. i

2. In a method of making tin plate comprising electrodepositing a layer of tin on sheet steel, heating the, tin plate to refiow the tin, cooling-the tin plate, making the plate cathode in an acid solution containing from .2 to 3% of a chromate of an alkali metal and having a pH value of from 2 to 7, thereby at least partially reducing the oxide film initially existing on the plate, and immediately thereafter making the plate anode while still in said with an aqueous solution containing 0.2 to 1.0% of a compound selected from the group consisting ofstannous chloride and citric acid, drying and oiling thetin plate, thereby reducing the tendency to eye holing on lacquerin'g.

References Cited in the file of thispatent I UNITED STATES PATENTS The temperature of the Neish Aug. 12, 1952 Neish Nov..5, 1957" 2,833,703 Laue May 6,1958, 

1. A METHOD OF MAKING TIN PLATE WHICH CONSISTS IN ELECTRODEPOSITING A COATING OF TIN ON A SHEET-STEEL BASE, FORMING A PROTECTIVE FLIM CONTANINIG CHROMIUM COMPOUNDS AND AT LEAST A RESIDUAL AMOUNT OF FREE HEXAVALENT CHROMIUM OVER THE TIN COATING AND THEN TREATING THE TIN PLATE WITH AN AQUEOUS SOLUTION CONTAINING FROM .2 TO 1% OF A REDUCING AGENT EFFECTIVE TO REDUCE ANY FREE HEXAVALENT CHROMIUM THEREON TO TRIVALENT CHROMIUM. 